1. Field of the Invention
The present invention relates to a control apparatus for an internal combustion engine in which the amount of intake air drawn into cylinders is changed by a variable intake mechanism thereof.
2. Description of the Related Art
Conventionally, for an internal combustion engine in which the amount of intake air drawn into cylinders is changed by a variable intake mechanism thereof, there has been proposed a control apparatus that controls the amount of fuel injection, in Japanese Laid-Open Patent Publication (Kokai) No. 2004-353480. As the variable intake mechanism, the engine is equipped with a variable valve mechanism which is comprised of an actuator, a control shaft that is axially driven by the actuator, and a swinging cam that is driven by the control shaft such that the cam swings about the axis of the control shaft. The swinging cam is provided between an intake cam and a rocker arm, and has an input part and an output part which are in contact with the intake cam and the rocker arm, respectively. In this variable valve mechanism, when the control shaft is axially actuated by the actuator, the input part and the output part of the swinging cam swing about the axis of the control shaft in different directions, whereby the lift of the intake valves (hereinafter referred to as “the valve lift”) and valve timing are changed.
In the variable valve mechanism described above, when the variable valve mechanism undergoes thermal expansion and contraction as the temperature of the variable valve mechanism changes, the valve lift and valve timing change, so that the accuracy of the fuel injection control degrades. Therefore, to carry out the fuel injection control while compensating for the influence of the thermal expansion and contraction of the variable valve mechanism, according to an example illustrated in FIGS. 8 to 11 of Japanese Laid-Open Patent Publication (Kokai) No. 2004-353480, the fuel injection amount is determined as follows:
First, target values of the valve lift and valve timing are determined according to the opening of an accelerator pedal, and the fuel injection amount is determined based on the valve lift and valve timing. Then, a temperature-dependent correction ratio is calculated according to the engine coolant temperature, and a timing correction ratio is calculated according to the target value of valve timing. The temperature-dependent correction ratio corrects or eliminates the influence of the thermal expansion and contraction of the variable valve mechanism on the intake air amount, while the timing correction ratio corrects a change in the degree of influence of the thermal expansion and contraction on the intake air amount, which is caused by a change in valve timing. Then, the fuel injection amount is multiplied by the two correction ratios to determine the final fuel injection amount.
In the case of the variable valve mechanism described above, the temperature thereof is not only changed by the influence of the engine coolant temperature but also by other temperature parameters, and hence the engine coolant temperature does not accurately or correctly reflect the degree of thermal expansion and contraction of the variable valve mechanism. What is more, after stoppage of the engine, the temperature of the variable valve mechanism sharply lowers due to dissipation of heat into the atmosphere, whereas the engine coolant temperature is more difficult to lower than the temperature of the variable valve mechanism due to the differences in heat capacity and specific heat, and hence when the engine is restarted after the stoppage, the correlation between the temperature of the variable valve mechanism and the engine coolant temperature becomes low. However, in carrying out the correction dependent on the temperature of the variable valve mechanism, the control apparatus proposed in Japanese Laid-Open Patent Publication (Kokai) No. 2004-353480 calculates the temperature-dependent correction ratio only based on the engine coolant temperature, and hence is incapable of properly performing the correction depending on the degree of thermal expansion and contraction of the variable valve mechanism, which degrades the accuracy of the control.
As a solution to the problem described above, it is contemplated as described in Paragraph Number [0022] of Japanese Laid-Open Patent Publication (Kokai) No. 2004-353480 that the temperature of the thermal expansion and contraction of the variable valve mechanism is directly detected by a temperature sensor or a strain sensor. However, this configuration requires a sensor for directly detecting the temperature of the thermal expansion and contraction of the variable valve mechanism to be provided separately in the vicinity of the variable valve mechanism of the engine, which brings about the problems of an increase in the size of the engine, a lowered degree of freedom of design, and an increase in the manufacturing costs.
Further, the variable intake mechanism including the variable valve mechanism suffers from the problem that due to other events than the thermal expansion and contraction, such as wear of components of the variable intake mechanism, attachment of stain, and play produced by aging, the intake air amount sometimes deviates from the proper value. In such a case, even when the degree of the thermal expansion and contraction of the variable intake mechanism is corrected, it is impossible to correct the deviation of the intake air amount caused by the other events than the thermal expansion and contraction, which can degrade the control accuracy.